1. Field of the Invention
The present invention relates to a liquid supplying device for supplying liquid to a consumption object that consumes the liquid (a liquid discharging head that discharges ink), a liquid discharging device, and a method of controlling the liquid discharging device. In addition, more specifically, the present invention relates to a technology that can easily stabilize the height of a liquid surface in an auxiliary tank that temporarily stores liquid prior to supplying the liquid to a consumption object that consumes the liquid.
2. Description of the Related Art
Hitherto, as an example of a liquid discharging device that discharges (consumes) a liquid, an inkjet printer that forms an image by supplying ink (the liquid) to a liquid discharging head and by discharging the ink towards a recording sheet from a nozzle of the liquid discharging head is available. In such an inkjet printer, for stably discharging the ink, it is demanded that the ink be stably supplied to the liquid discharging head from an ink cartridge (ink tank) that stores the ink.
For stably discharging the liquid from the inkjet printer, it is necessary for the pressure of the ink in the liquid discharging head to be slightly negative with respect to atmospheric pressure. When the negative pressure is too low, the ink tends to leak from the nozzle, and the ink is unstably discharged. In contrast, when the negative pressure is too high, a proper amount of ink is not discharged, and the ink discharge becomes unstable.
Therefore, for maintaining a proper negative pressure in the liquid discharging head, in general, a porous material (such as urethane foam) is accommodated in the ink cartridge, and negative pressure is generated by capillarity of the porous material. A method of generating negative pressure by storing the ink in a flexible ink bag and pulling the ink bag by, for example, a spring is also available.
However, in recent inkjet printers, for achieving a high speed, the number of nozzles tend to be increased. In particular, since, in a line-head inkjet printer, a plurality of nozzles are disposed in accordance with the width of a recording sheet having the largest size that can be printed, the number of nozzles is very large compared to that of a serial head printer that performs printing by being moved in a sheet width direction. Therefore, the amount of consumption of ink per unit time is large, and, in the method that generates negative pressure by a porous material, there is not enough ink that can be supplied. In addition, in the method that pulls a flexible ink bag, a large spring is used for reducing changes in negative pressure, thereby increasing the size of the inkjet printer.
Due to such circumstances, a technology in which ink is temporarily stored in a subtank (auxiliary tank) prior to supplying the ink to the liquid discharging head and in which the subtank is disposed below a nozzle to maintain a negative pressure on the basis of a water head difference is provided. If an ink holding volume of the subtank is sufficiently ensured, and the height of an ink liquid surface in the subtank is maintained in a predetermined range, the pressure of the ink in the liquid discharging head becomes a negative pressure in a proper range allowing a meniscus to be formed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-132762 (Patent Document 1)).